Quantitative amino acid analysis of feedstuff hydrolysates by reverse phase liquid chromatography and conventional ion-exchange chromatography

Corn, soybean meal, and isolated soybean protein samples were acid-hydrolyzed for amino acid content by reverse phase liquid chromatography (LC) and by conventional ion-exchange chromatography (IEC) using an amino acid analyzer. The former method employed pre-column derivatization with orthophthalaldehyde (OPTA)/ethanethiol and fluorescence detection. In the LC procedure, glycine and threonine were not resolved, and proline and cyst(e)ine were not detected. In general, amino acid values obtained by LC and IEC compared closely within and across feedstuffs, and both agreed well with published amino acid composition data. The notable exceptions were aspartic acid, glutamic acid, and alanine. Results of this study suggest that reverse phase LC with pre-column OPTA derivatization can be applied to accurately measure primary amino acids in individual feedstuffs.